1. Field of the Invention
The present invention relates to a water base ink for ink-jet recording which has satisfactory purge restoration performance while securing a high recording quality with a sharp image area edge, a method for producing the same, and an ink-jet recording apparatus which is provided with the ink.
2. Description of the Related Art
In the ink-jet recording system, ink droplets are formed by means of the ink discharge method including, for example, the electrostatic attraction method, the method in which mechanical vibration or displacement is applied to the ink by using a piezoelectric element, and the method in which bubbles are generated by heating the ink to utilize the pressure generated thereby. All or a part of the ink droplets are adhered to a recording objective material such as paper to perform the recording.
Those having been hitherto developed as the ink for ink-jet recording to be used for the ink-jet recording method as described above include the oil base ink which uses an organic solvent as a solvent and the water base ink which uses water as a main solvent. However, in recent years, the development is dominantly directed to the water base ink in consideration of the safety and the environment.
When the recording is performed with an ink-jet printer by using a general water base ink, any exclusive ink-jet paper is sometimes used in order to obtain a satisfactory printing quality without any blurring of the ink. However, in recent years, it is more demanded that the recording is performed on the regular paper rather than on the exclusive ink-jet paper, in consideration of the running cost and the environment.
When the recording is performed on the regular paper, a problem arises such that the recording quality is extremely deteriorated as compared with the case in which the recording is performed on the exclusive ink-jet paper. That is, the following problem arises. The edge of the image area is notched due to the phenomenon generally called “blurring” in which the ink is spread along the surface of the paper when the ink is permeated into the paper. As a result, it is impossible to obtain any sharp edge of the image area. For this reason, a method is generally used, in which the blurring is suppressed to obtain a sharp image area edge by making the surface tension of the ink to be not less than 40 mN/m.
On the other hand, as shown in FIG. 1, an ink-jet recording apparatus has four recording heads 101 which are mutually aligned and arranged on a holder 104 disposed on a carriage 103. Respective ink cartridges 102 of four colors are detachably installed to the holder 104 corresponding to the four recording heads 101. The carriage 103 is slidably supported by guide rails 107, 108 so that the carriage 103 is movable in the widthwise direction of recording paper 105 in a state in which the respective recording heads 101 are opposed to the recording paper 105. The carriage 103 is reciprocatively moved in the foregoing direction by a belt 112 which is stretched between a pulley 111 and a pulley 110 that is driven by a motor 109. The recording paper 105 is transported by a transport roller 106 in a direction perpendicular to the direction of the movement of the carriage. The respective recording heads 101 discharge ink droplets toward the recording paper at positions opposed to the recording paper 105 to perform the recording. A restoring unit 113 is provided at a position at which the restoring unit 113 is opposed to the recording head 101 when the carriage 103 is moved to a position outside of the recording paper 105. As disclosed, for example, in Japanese Patent Application Laid-open No. 10-250113, the restoring unit 113 comprises a suction cap 114 which is capable of making contact with the nozzle surface of the recording head 101, and a suction pump 115 which sucks the ink contained in the recording head through the suction cap 114. The suction cap 114 is moved in the direction to make approach or separation with respect to the recording head 101 by a cam 116, and the suction pump 115 is driven to effect the suction. The restoring unit performs the so-called purge operation in which the suction cap 114 is brought into contact with the nozzle surface of the recording head 101 to suck the ink contained in the recording head when the ink cartridge 102 is exchanged to introduce a new ink into the ink flow passage in the recording head or when any dust or the like adhered to the inside of the nozzle of the recording head or the viscosity of the ink is increased resulting in the necessity to remove such matters. The sucked ink is discharged to a drain ink tank 117. Storing caps 118 are provided at positions opposed to the four recording heads 101 when the carriage 103 is further moved to the outside of the restoring unit 113. The storing caps 118 cover the nozzle surfaces of the four recording heads 101 to avoid any drying of the inks contained in the nozzles of the recording heads when the recording operation is not performed. When the purge operation is performed by effecting the suction from the side of the head nozzle by using the restoring unit as described above, any turbulent flow occurs, because the ink is allowed to flow at a high speed through the ink flow passage. Consequently, bubbles are generated from gas components which have been dissolved in the ink. Bubbles also enter the ink flow passage by contaminating the ink flow passage with the air coming through the connecting portion between the ink cartridge and the head unit when the ink cartridge is exchanged. When the bubbles contained in the ink adhere to the wall surface of the ink flow passage and the bubbles remain, then the ink is not discharged from the ink-jet nozzle and/or the discharge operation becomes unstable. As a result, the recording quality is extremely deteriorated.
In view of the above, in order to discharge the bubbles from the ink flow passage, a method is used, in which the wettability between the ink and the wall surface of the ink flow passage is sufficiently improved by sufficiently lowering the surface tension of the ink to be less than 40 mN/m. When the wettability between the ink and the wall surface of the ink flow passage is sufficiently improved, then the ink promptly enters the interstices between the wall surface of the ink flow passage and the bubbles, and the bubbles are exfoliated from the wall surface of the ink flow passage so that the bubbles are discharged together with the ink, even when the bubbles inflow into the ink flow passage or when the bubbles are generated in the ink flow passage. Therefore, the bubbles scarcely remain on the wall surface of the ink flow passage. In other words, it is considered that the force, which is exerted to effect the exfoliation in this case, is larger than the force of the bubbles to adhere to the wall surface.
It is noted that the ink, in which the surface tension is not less than 40 mN/m in order to suppress the blurring on the recording medium and obtain the sharp image area edge, is inferior in wettability between the ink and the wall surface of the ink flow passage. Even when the purge operation is performed in order to discharge the bubbles from the ink flow passage, then the bubbles are not exfoliated while adhering to the wall surface of the ink flow passage and the bubbles are hardly discharged, because it is difficult for the ink to enter the interstices between the wall surface of the ink flow passage and the bubbles, probably for the following reason. That is, in this case, it is considered that the force, which is exerted to effect the exfoliation, is smaller than the force of the bubbles to effect the adhesion.
That is, when the surface tension of the ink is increased in order to obtain the sharp image area edge, the purge restoration performance is deteriorated. On the other hand, when the surface tension of the ink is lowered in order to improve the purge restoration performance, the sharp image area edge is not obtained. Therefore, the following problem has arisen. That is, it is extremely difficult to satisfy both of the sharp image area edge and the satisfactory purge restoration performance.